The basement of the WAC was built through several major orogenic cycles: the Paleoarchean–Leonian cycle (multiple episodes between 3.5 and 3.0 Ga) related to continental accretion and volcano-sedimentary activity whose chronology remains uncertain (e.g., Thiéblemont et al., 2004), the Liberian cycle (2.95–2.75 Ga; Key et al., 2008), the Eburnian–Birimian cycle (2.2–1.75 Ga; Schofield et al., 2006) and the Pan-African orogenic event (760–660 Ma; Thomas et al., 2002). One of the main characteristics of the WAC is that no Mesoproterozoic events or rocks are known, suggesting a quiescent period between 1.7 and 1.0 Ga (e.g., Ennih and Liégeois, 2008).
They all consist essentially of tholeiitic basalts and are typically composed of plagioclase + clinopyroxene (augite) ± orthopyroxene (enstatite) ± olivine displaying doleritic texture and variable grain size. (2006) Short-lived mantle generated magmatic events and their dyke swarms: The key unlocking Earth's paleogeographic record back to 2.6 Ga.
From the geochemical point of view, they are all characterized by a clear negative Nb anomaly, indicating a plausible subduction fingerprint and/or crustal contamination (e.g., Bassot et al., 1986; Ikenne et al., 1997; Hafid et al., 2001; Verati et al., 2005 Deckart et al., 2005; Chabou et al., 2010; Cournède, 2010). In: Dyke Swarms-Time Markers of Crustal Evolution, edited by E.
The similarities of all their trace element patterns play in favor of a single geodynamic environment or a common influence of contamination from sub-lithospheric mantle.
Thus, the study of dyke rocks is extremely important in order to reveal the configuration of ancient supercontinents.
In this contribution we present a preliminary survey of mafic dyke swarms of the West African craton (WAC).
The synthesis demonstrates the complexity in the dyke record, suggesting that geochronological, petrological, geochemical and paleomagnetic characteristics of many dyke sets remain to be established before WAC and its shield areas can be put into a global plate tectonic framework (e.g., Youbi et al., 2010). The WAC, stable since 2 Ga, constitutes the basement of northwestern Africa (e.g., Ennih and Liégeois, 2008). Palaeomagnetic and radiometric evidence for the age of the Freetown igneous complex, Sierra Leone. The WAC is composed of three Archean and Paleoproterozoic shields: the Reguibat Shield, the Anti-Atlas and the Leo-Man shield, separated by two cratonic sedimentary basins. Petersburg, Rome 2015: Brisbane, Indian Wells, Miami, Charleston, Wimbledon, U. Open, Guangzhou, Wuhan, Beijing, WTA Finals 2014: Miami, Wuhan, Moscow 2007: Doha 2002: Australian Open, Hamburg 2001: Moscow 2000: Roland Garros, Tokyo, Filderstadt, Zürich, Philadelphia, WTA Finals, Montréal 1999: Australian Open, Indian Wells, Rome, Eastbourne, WTA Finals, Miami 1998: Australian Open, Tokyo, Miami, Roland Garros, Wimbledon, Montréal, U. Dept of Geology, Faculty of Sciences, Ibnou Zohr University, P. Box 28/S, Agadir, Morocco, Email: [email protected] Introduction Regional dyke swarms are considered to represent periods of repeated crustal extension wherein enormous amounts of mantle-derived magmas ascend through the crust. Their distribution, palaeomagnetism, petrology, geochemistry and emplacement ages are of intense geodynamic interest since they provide invaluable clues to our understanding of the nature of the sub-continental lithospheric mantle. There is also an increasing global recognition of their utility in delineating the existence and extent of Large Igneous Provinces (LIPs) and, especially, to use their exact emplacement ages and dyke directions to reconstruct formerly adjacent crustal blocks (e.g., Bleeker and Ernst, 2006; Ernst and Bleeker, 2010). P., Thiéblemont, D., Le Metour, J., Egal, E., Donzeau, M., Guerrot, C., Cocherie, C., Chevremont, P., Tegyey, M., Itard, Y., Zida, B., Ouedraogo, I, Kote, S., Kabore, B.